That is caused by your leaves laying on top of medium. Try to keep them off if you can. But nothing to be alarmed about. Everything you said is on point even 21c that is perfect water for hydro. Just be careful with nutrients cuz autos don't feed as much as a photoperiod. They use far less nutrients. I start autos at 1/8 strength and Max at around 1/3 strength of manufacturer recommendation.
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